1. Field of the Invention
The present invention relates to a fuse circuit and, particularly, to a fuse circuit which determines the cut state of a fuse by the operation of a current mirror circuit.
2. Description of Related Art
The number of devices that are formed on one semiconductor chip is ever increasing This causes difficulty in manufacturing all devices without any defect, which leads to a decrease in manufacturing yield. In light of this, there is proposed a semiconductor chip that replaces a defective device with a redundant device which is formed on the same chip to relieve the defective chip. The circuit in which such replacement is made uses a state storage device such as a fuse and selects whether to use an existing device or a redundant device according to the uncut/cut state of the device. Further, such a circuit uses a fuse circuit which determines the uncut/cut state of a fuse. An example of the fuse circuit is disclosed in Japanese Unexamined Patent Application Publication No. 2005-332964 (which is hereinafter referred to as a related art).
FIG. 18 shows a circuit diagram of a fuse circuit 100 which is described in the related art. Referring to FIG. 18, the fuse circuit 100 includes transistors Q1-Q6, which are grouped as circuits 110-112, to produce currents I11-I14 and an inverter X1. The current I12 and current I14 which are supplied to an output terminal DET through current mirror circuits 111 and 112. The fuse circuit 100 sets the magnitude relationship between the current I12 and the current 114 based on the resistance ratio between a fuse F10 and a resistor R10. When the fuse F10 is uncut, the resistance of the fuse F10 is sufficiently lower than the resistance of the resistor R10, and when it is cut, the resistance of the fuse F10 is sufficiently higher than the resistance of the resistor R10. Accordingly, when the fuse F10 is uncut, the current I14 is larger than the current I12, and when it is cut, the current I12 is larger than the current I14. In this way, the magnitude relationship between the current I12 and the current I14 is reversed according to the uncut/cut state of the fuse F10, so that the potential of the output terminal DET is inverted. A circuit in the subsequent state can thereby determine the uncut/cut state of the fuse from the potential of the output terminal DET.
However, the fuse F10 can be connected again after it is cut in some cases. If the reconnection occurs in the fuse F10, the resistance of the fuse F10 becomes lower than the resistance when it is cut. Accordingly, the current I12 and the current I14 may not have a desired magnitude relationship in some cases where the resistance of the fuse F10 after the reconnection and the resistance of the resistor R10 are in certain relationship. Further, the resistance of the resistor R10 varies in the manufacturing process. If the resistance varies significantly, the relationship between the resistance of the fuse F10 and the resistance of the resistor R10 does not have a desired ratio. Due to such issues, there is the probability of wrong determination of the uncut/cut state of the fuse F10 in the fuse circuit 100.